The present application claims priority to Japanese Patent Application No. 11-142611 filed May 24, 1999, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to an optical waveguide channel device, and more particularly, to a multi-beam light source that is suitable for use as a light source in a laser printer.
2. Description of the Related Art
In recent years, as the information network has become increasingly advanced and digital, faster laser printers have come into strong demand. One means to speed up a laser printer involves acceleration of the rotation of the polygon mirror used for scanning. However, when the rate of rotation of the polygon mirror approaches 50,000 rpm, distortion is caused in the polygon surfaces due to the centrifugal force, and therefore it is difficult to increase the rate of rotation of the polygon mirror beyond the current level. Consequently, in order to further increase the image draw speed of a laser printer, scanning of the photoreceptor surface using multiple laser beams has conventionally been used.
Specifically, as disclosed in Japanese Laid-Open Patent Application Hei 10-282441, U.S. Pat. No. 4,637,679, U.S. Pat. No. 4,547,038 and U.S. Pat. No. 4,958,893, constructions have been proposed or adopted in which multiple laser beams are adjusted to have appropriate intervals between them by employing reflection by a polarized light beam splitter, half-mirror and/or prism surfaces to optically deflect the multiple laser beams. However, these methods have the shortcoming that if the number of the laser beams is large, it is difficult to appropriately align them, leading to large components and excessive cost. As a result, it is very difficult to speed up a laser printer beyond the current level using these methods.
Against this backdrop, a method in which a so-called multi-beam light source comprising multiple laser beams aligned according to a minute pitch is desired. As a specific realization of this method, three methods have been proposed, as disclosed in Japanese Laid-Open Patent Application Sho 54-7328: a method in which a so-called array laser comprising multiple laser diodes on a substrate is used as a multi-beam light source; a method in which the light emitted from optical fibers is used as a secondary light source; and a method in which an optical waveguide channel device equipped with multiple cores whose pitch at the light exit edge is smaller than that at the light entry edge.
In the method using an array laser, considering the state of image formation on the photoreceptor surface, it is preferred that the alignment pitch of the laser diodes be very small at 100 xcexcm or less, so that the multiple laser beam spots are sufficiently close to one another. However, forming laser diodes on a substrate using such a minute pitch entails the problem of heat generation, and is therefore difficult. Therefore, the other methods involving the use of optical fibers or an optical waveguide channel device would appear to be more effective. Among those, the type that uses an optical waveguide channel device has the characteristic that it offers a high level of freedom in making the multiple optical channels small. In this patent application, an optical waveguide channel device refers to a flat waveguide channel device formed on a substrate.
Where a multi-beam light source is constructed using an optical waveguide channel device, it is difficult to cause multiple laser beams to enter the device at the entry edge while close to each other, and therefore, multiple laser beams are caused to strike the entry edge with certain intervals in between. These multiple laser beams need to be closer together at the exit edge. In other words, the distances between the axes of the multiple cores in which the laser beams pass in the optical waveguide channel device must be smaller at the exit edge than at the entry edge. In order to achieve this, the cores conventionally have had curved areas in which the axes are curved.
FIG. 1 is a cross-section of one example of a conventional multi-beam light source. In the light source 51, multiple semiconductor laser devices 52 are used. Cores 54 of the optical waveguide channel device 53 are connected to the semiconductor laser devices 52 via lenses 55. As shown in FIG. 1, in the optical waveguide channel device 53, each core 54 is curved at multiple points (54a, 54b) in order to make the distances between the cores 54 smaller at the exit edge 53b than at the entry edge 53a. 
It is generally known that, where laser beams are input and connect with the optical waveguide channel device from the entry edge, in order to increase the connecting efficiency, laser beams having a beam profile similar to the beam profile of the core""s waveguide mode should be made to enter the device such that their optical axes match each other. However, the core diameter in an optical waveguide channel device generally small, and where it has a single waveguide mode, the core diameter could be as small as several micrometers. Therefore, in order to increase the connecting efficiency, the beam spots having a diameter of only several micrometers must be caused to enter the device, which makes it difficult to match the optical axes of the beam and the core. Further, if the optical axes do not match even by a small degree, the connecting efficiency is markedly reduced. If a discrepancy occurs between the optical axes due to diminished accuracy in position adjustment or aging of the device, the quality and power of the exit light is reduced to the degree that it pauses a practical problem.
Therefore, the core diameter in the optical waveguide channel device and the beam spot diameter of the incident light are increased. This not only makes positional adjustment easier but also reduces the rate of reduction of the connecting efficiency due to non-matching optical axes, increasing the tolerance for any discrepancy between the optical axes. The waveguide mode for a large diameter core is generally multi-mode.
However, in a multi-mode core that has a curved area, high-order light is easily excited in the curved area. Consequently, even if basic mode light is caused to enter the device, it is difficult to obtain exit light having a Gaussian profile. If the radius of curvature of the curved area were increased, excitation of high-order mode light that occurs in the curved area might be reduced, but the device would be large in size.
The object of the present invention is to provide an optical waveguide channel device having a construction in which the distances between the cores are smaller at the exit edge than at the entry edge, and in which the tolerance of any optical axis discrepancy when the laser beam and the core are connected may be increased and the light loss amount is small.
Another object of the present invention is to provide an optical waveguide channel device comprising a multi-beam light source from which exit light having an essentially Gaussian profile may be obtained.
These and other objects may be attained by an optical waveguide channel device, comprising a substrate; and multiple cores that are formed on the substrate and guide coherent beams, the distances between the axes of adjacent cores being smaller at the exit edge than at the entry edge, wherein a single mode is present in a specific area from the entry edge of each core to the exit edge, and the area of the cross-section perpendicular to the axis of the core is larger at the entry edge than at the specific area.
In the optical waveguide channel device having the construction described above, the area of each core at the entry edge is increased, and therefore, it is easy to allow the coherent beam to enter the core within the range of optical axis discrepancy tolerance. In addition, it is likely that the optical axis discrepancy that occurs due to aging will remain within the tolerance range. Further, by designating the area of each core at which high-order mode light is easily excited as the specific area and having the waveguide mode in that area be single mode, a construction in which high-order mode light is not easily excited may be obtained. Therefore, exit light having an essentially Gaussian beam profile with a small light loss may be obtained.
By having a curved area in each core, it becomes possible to easily construct an optical waveguide channel device in which the distances between the axes of the cores are smaller at the exit edge than at the entry edge. However, because high-order mode light is easily excited in the curved area of the core, by having the waveguide mode in this area be single mode, the excitation of high-order mode light is reduced.
It is also preferred that the area of the cross-section perpendicular to the axis of each core be different at the exit edge and within the curved area. Generally, the beam spot diameter and the expansion angle of the exit light vary depending on the cross-sectional area of the core at the exit edge. Therefore, by having the cross-sectional area of the core match the required exit light, any desired exit light may be obtained. On the other hand, because transmission in the curved area takes place in a single mode, the profile of the exit light is essentially Gaussian.
It is also preferred that the core has a prescribed area having a straight axis and a consistent configuration and an area in the cross-section perpendicular to the axis immediately before or after the curved area. Where the axis of the core is curved, when light travels, the profile becomes asymmetrical. Therefore, by creating immediately after the curved area a straight area in which force exhibiting this asymmetry does not operate, the asymmetry in the force received in the curved area is reduced. Alternatively, by having a straight area immediately before the curved area, the traveling light that is not receiving any asymmetrical force enters the curved area, so that high-order light does not easily result in this area.